ABSTRACT
An international collaborative study of breast cancer and reproductive experience was carried out in seven areas of the world. In all areas studied, a striking relation between age at first birth and breast cancer risk was observed. It was estimated that women having their first child when aged under 18 years have only above one-third the breast cancer risk of those whose first birth is delayed until the age of 35 years or more. Births after the first, even if they occur at an early age, have no, or very little, protective effect. The reduced risk of breast cancer in women having their first child at an early age explains the previously observed inverse relationship between total parity and breast cancer risk, since women having their first birth early tend to become ultimately of high parity. The association with age at first birth requires different kinds of aetiological hypotheses from those that have been invoked in the past to explain the association between breast cancer risk and reproductive experience. Almost all subsequent work on breast cancer aetiology cites this or derivative publications.
